One assumption I make, which I'm guessing you don't, is that rules for PC generation only apply to PCs, and the logic in the chargen rules...specifically the attribute rules...aren't meant to be extrapolated to mean anything. If the rules allow for a PC halfling with 20 strength, but nobody in your group actually makes one, then there are no 20 strength halflings in the world. Or, at least, if the DM decides there are no 20 strength halflings in the world...if there aren't even any halflings with above 13 strength...there's no contradiction. Because those rules are metagame rules that exist solely for the creation of PCs, and do not describe wider statistical patterns.
If you and your group further agree that no PC halfling should be as strong as a goliath halfling, then that won't happen either.
If somebody doesn't agree, and shows up with a super strong halfling, and that really bothers you, then one of you is in the wrong gaming group.